The present invention is directed to an optical sensor which may be used in conjunction with a radio frequency (rf) sensor as part of a dual mode sensor. More particularly, this invention relates to a dual mode millimeter wave and optical sensor system for use with a guidance system of a projectile.
Dual mode sensors are increasingly employed in conjunction with the guidance systems of munitions to improve operational flexibility and resistance to countermeasures. A number of approaches have been taken in designing such dual mode sensors.
In one approach called `common aperture` and often seen in aperture-limited applications such as munitions and covert sensors, a common aperture is shared by the radio frequency (rf) sensor and the optical sensor so as to allow each sensor to collect the maximum incoming energy. To that end, U.S. Pat. No. 5,214,438 discloses a millimeter wavelength (mmw) and infrared sensor having a common receiving aperture. U.S. Pat. No. 4,866,454 similarly discloses a multi-spectral imaging system having a common aperture. Other representative patents include U.S. Pat. No. 4,652,885 and U.S. Pat. No. 4,636,797 which both disclose a dual mode antenna in which mmw and infrared energy enter a common aperture and propagate through a common transmission device to a point where the energies of respective wavelength are divided for subsequent processing. "Common Aperture Dual Mode Semi-Active Laser/Millimeter Wave Sensor" a U.S. patent application Ser. No. 08/959,602 filed Oct. 28, 1997, now U.S. Pat. No. 5,973,649, the entire contents of which are incorporated herein by reference describes yet another approach to a dual mode single aperture sensor which uses a pyramid with surfaces selectively coated to reflect semi-active laser energy and selectively passes mmw energy.
Unfortunately, common aperture dual mode sensors typically result in decreased sensitivity and decreased accuracy for both modes. Elements that are used by both the mmw and optical sensors must either be broadband to encompass both operating wavelengths thereby decreasing sensitivity or have selective coatings to selectively pass one wavelength while reflecting the other. Where it is desired to use such dual mode sensors on high speed munitions, the design problems are exacerbated as many of the coatings and other materials useful for single aperture constructions are not robust enough to operate at high speeds where frictional heating becomes significant.
In another approach to the dual mode sensor called `separate aperture`, separate apertures are employed for each wavelength band. U.S. Pat. No. 5,182,564 to Burkett et al. discloses one such device in which a mmw rf microstrip array is embedded within an infrared reflector assembly. U.S. Pat. No. 4,264,907 to Durand, Jr. et al. discloses a seeker system on a missile comprising an infrared sensor and an rf sensor. The dual mode seeker employs two rf antennae emanating from the missile. U.S. Pat. No. 4,108,400 to Groutage et. al. also discloses a dual mode guidance system having two separate apertures for detecting energy. The rf sensors are disposed on the skin of a missile about an alternate sensor. U.S. Pat. No. 4,698,638 to Branigan et al. discloses a dual mode target seeking system comprising an rf antenna and an electro-optical processing system extending through the rf antenna.
Where wavelengths in excess of mmw are detected, an optical sensor may be placed in front of the rf sensor without significantly interfering in the operation. In the case of mmw, however, the blockage of the center of the rf sensor by the electro-optical sensor can degrade the resolution of the rf sensor. While this problem can be minimized by reducing the size of the optical sensor, this in turn reduces the sensitivity and range of the optical sensor.
It is a goal of the present invention to provide an unfocused optical sensor which may be used in conjunction with an rf sensor as part of a dual mode sensor that overcomes the problems of the prior art devices. The inventive dual mode sensors have an inner mmw rf sensor surrounded by an unfocused optical sensor and can be used in conjunction with the guidance system of a munition.
For the purposes of this disclosure, the term `munition` is to be understood in its generic sense which includes projectiles, and ammunition including bullets, missiles and rockets. Also, for the purposes of this application, the term `target source of optical energy` shall refer to any source of reflected optical energy, i.e. any optical energy reflected off of a target.